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  Preparation of Graphene Selected Topics in Physics: Physics of Nanoscale CarbonNils Krane Nils.Krane@fu-berlin.deTodaythereareseveralmethodsforthepreparationofgraphene. Inthefollowingsomeofthesemeth-odswillbepresentedanddiscussed. Theywillbecomparedusingspecificrequirementsandallocatedto different purposes. The requirements are e.g. quality and size of the flakes or controllability of theresulting coating. 1 Introduction Graphene is a very special material, since ithas the advantage of being both conducting andtransparent. Thetransparencyofamate-rial normally depends on its electronic prop-erties and requires a band gap. Under nor-mal conditions transparency and conductiv-ity exclude each other, except for a few com-pounds like indium tin oxide (ITO). How-ever, in contrast to ITO, graphene is also flex-ible and capable of withstanding high stress.Therefore it is very attractive for the applica-tion of flexible electronic devices, e.g. touchscreens [1]. Accordingly, there are a lot of ef-fortsinordertopreparegrapheneeasylywiththe required properties.The methods described in this review areevaluated on the bases of different require-ments: On the purity of the graphene, whichis defined by the lack of intrinsic defects,( Quality  )aswellasonthesizeoftheobtainedflakes or layers ( Size  ). Another aspect is theamount of graphene which can be producedsimultaneously (  Amount  ) or the complexity suchastherequirementoflabourortheneedfor specially designed machines ( Complex. ).One last attribute is the controllability of themethod in order to achieve reproducible re-sults ( Control. ).This review is organized as follows: In Sec.2 the exfoliation methods are presented anddiscussed. Following that another type of preparation, the growth of graphene, will beintroduced in Sec. 3. Concluding, the differ-ent methods will be compared in Sec. 4. 2 Exfoliation Basically there are two different approachesto preparing graphene. On the one handgraphene can be detached from an already existing graphite crystal, the so-called ex-foliation methods, on the other hand thegraphene layer can be grown directly on asubstrate surface. The first reported prepara-tion of graphene was by Novoselov and Gaimin 2004 [2] by exfoliation using a simple ad-hesive tape. 2.1 The “Scotch Tape Method” In this micromechanical exfoliation method,graphene is detached from a graphite crys-tal using adhesive tape. After peeling itoff the graphite, multiple-layer graphene re-mains on the tape. By repeated peeling themultiple-layer graphene is cleaved into vari-ous flakes of few-layer graphene. Afterwardsthe tape is attached to the substrate and theglue solved, e.g. by acetone, in order to de-tachthetape. Finallyonelastpeelingwithanunused tape is performed.The obtained flakes differ considerably insize and thickness, where the sizes rangefromnanometerstoseveraltensofmicrome-ters for single-layer graphene, depending onthe preparation of the used wafer. Single-1  layer graphene has a absorption rate of 2%,nevertheless it is possible to see it under alight microscope on SiO 2 /Si, due to interfer-ence effects [3]. However, it is difficult toobtain larger amounts of graphene by thismethod, not even taking into account thelack of controllability. The complexity of this method is basically low, nevertheless thegrapheneflakesneedtobefoundonthesub-strate surface, which is labour intensive. Thequality of the prepared graphene is very high with almost no defects. 2.2 Dispersion of Graphite Graphene can be prepared in liquid-phase.This allows upscaling the production, in or-der to obtain a much higher amount of graphene. The easiest method would beto disperse the graphite in an organic sol-vent with nearly the same surface energy asgraphite[4]. Thereby,theenergybarrierisre-duced, which has to be overcome in order todetachagraphenelayerfromthecrystal. Thesolution is then sonicated in an ultrasoundbath for several hundreds hours or a voltageis applied [5]. After the dispersion, the solu-tion has to be centrifuged in order to disposeof the thicker flakes.The quality of the obtained grapheneflakes is very high in accordance with the mi-cromechanical exfoliation. Its size howeveris still very small, neither is the controllabil-ity given. On the other hand, the complex-ity is very low, and as mentioned above thismethod allows preparing large amounts of graphene. 2.3 Graphite Oxide Exfoliation The principle of liquid-phase exfoliation canalso be used to exfoliate graphite oxide. Dueto several functional groups like epoxide orhyroxyl, graphene oxide is hydrophilic andcan be solved in water by sonication or stir-ring. Thereby the layers become negatively charged and thus a recombination is inhib-ited by the electrical repulsion. After cen-trifugation the graphene oxide has to be re-Figure 1:  (a)  Solution of graphene in liquid-phase. Theflaskscontainsolutionsaftercen-trifugation at different frequencies [4].  (b) Scheme of the exfoliation of graphite ox-ide. The graphite gets oxidized and solvedin water. Afterwards it gets reduced tographene [6].duced to regular graphene by thermal orchemical methods. It is hardly possible todispose of all the oxygen. In fact, an atomicC/O ratio of about 10 still remains [6].The performance of this method is very similar to liquid-phase exfoliation of pristinegraphene. Only the complexity is higher,since graphite oxide has to be producedfirst, wich requires the use of several chem-icals. Also the obtained graphene oxidehas to be reduced afterwards, using thermaltreatments or chemicals again [7]. The re-duced graphene oxide is of very bad quality compared to pristine graphene, neverthelessgraphene oxide could be the desired prod-uct. Graphene oxide modified with Ca andMg ions is capable of forming very tensilegraphene oxide paper, as the ions are cross-linkers between the functional groups of thegraphene flakes [8]. 2.4 Substrate Preparation There are different methods for substratepreparation in order to use the dispersedgraphene in a non liquid-phase. By vacuum2  filtration the solution is sucked through amembraneusingavacuumpump. Asaresultthe graphene flakes end up as filtration cakeof graphene paper.The deposition of graphene on a surfacecan be done by simple drop-casting where adrop of the solution is placed on top of thesubstrate. Afterthesolventshaveevaporated,thegrapheneflakesremainonthesurface. Inorder to achive a more homogeneous coat-ing the sample can be rotated using the spin-coating method in order to disperse the so-lution with the help of the centrifugal force. With spray-coating, the solution ist sprayedonto the sample, which allows the prepara-tion of larger areas. 3 Growth on Surfaces  A totally different approach to obtaining graphene is to grow it directly on a surface.Consequently the size of the obtained lay-ers are not dependent on the initial graphitecrystal. Thegrowthcanoccurintwodifferent ways. Either the carbon already exists in thesubstrate or it has to be added by chemicalvapour deposition (CVD). 3.1 Epitaxial Growth Graphene can be prepared by simply heating and cooling down an SiC crystal [9]. Gen-erally speaking single- or bi-layer grapheneforms on the Si face of the crystal, whereasfew-layer graphene grows on the C face [10].The results are highly dependent on the pa-rameters used, like temperatur, heating rate,or pressure. In fact, if temperatures and pres-sure are too high the growth of nanotubes in-stead of graphene can occur. The graphitiza-tion of SiC was discovered in 1955, but it wasregarded as unwelcome side effect instead of a method of preparing graphene [11].The Ni(111) surface has a lattice structurevery similar to the one of graphene, with amissmatch of the lattice constant at about1.3% [11]. Thus by use of the nickel diffu-sion method a thin Ni layer is evaporatedonto a SiC crystal. Upon heating the car-bon diffuses through the Ni layer and formsa graphene or graphite layer on the surface,depending on the heating rate. The thusproduced graphene is easier to detach fromthe surface than the graphene produced by the growth on a simple SiC crystal withoutNi [11].Figure 2: SEM image of graphene on cop-per foil. At several locations on the sur-face graphene islands form and grow to-gether [14].The growth of graphene starts at severallocations on the crystal simultaneously andthese graphene islands grow together, asshown in Fig. 2). Therefore the graphene isnotperfectlyhomogeneous,duetodefectsorgrain boundaries. Its quality therefore is notasgoodasthatofexfoliatedgraphene,exceptthe graphene would be grown on a perfectsingle crystal. However, the size of the ho-mogeneous graphene layer is limited by thesizeofthecrystalused. Thepossibilitytopro-duce large amounts of graphene by epitaxialgrowthisnotasgoodasbyliquid-phaseexfo-liation, though the controllability to gain re-producibleresultsisgiven. Alsothecomplex-ity of these methods is comparatively low. 3.2 Chemical Vapour Deposition Chemical vapour deposition is a well knownprocess in which a substrate is exposed togaseous compounds. These compounds de-compose on the surface in order to grow athinfilm,whereastheby-productsevaporate.3  Figure 3:  (a) Scheme of preparation of graphene by CVD and transfer via polymer support.The carbon solves into the Ni during the CVD and forms graphene on the surface after cool-ing. Withapolymersupportthegraphenecanbestampedontoanothersubstrate,afteretch-ingoftheNilayer. PatterningoftheNilayerallowsacontroloftheshapeofthegraphene[12]. (b)  Roll-to-roll process of graphene films grown on copper foils and transferred on a targetsubstrate [1].There are a lot of different ways to achievethis, e.g. by heating the sample with a fil-ament or with plasma. Graphene can begrown by exposing of a Ni film to a gas mix-ture of H 2 , CH 4  and Ar at about 1000°C [12].The methane decomposes on the surface, sothat the hydrogene evaporates. The carbondiffuses into the Ni. After cooling down in an Ar atomosphere, a graphene layer grows onthe surface, a process similar to the Ni diffu-sion method. Hence, the average number of layers depends on the Ni thickness and canbe controlled in this way. Furthermore, theshape of the graphene can also be controlledby patterning of the Ni layer.These graphene layers can be transferedvia polymer support, which will be attachedonto the top of the graphene. After etching the Ni, the graphene can be stamped ontothe required substrate and the polymer sup-port gets peeled off or etched away. Using this method several layers of graphene canbe stamped onto each other in order to de-crease the resistance. Due to rotation rel-atively to the other layers, the turbostraticgraphite does not have the Bernal stacking and consequently the single graphene lay-ers hardly change their electronic properties,since they interact marginally with the otherlayers [1].Using copper instead of nickel as grow-ing substrate results in single-layer graphene with less than 5% of few-layer graphene, which do not grow larger with time [13]. Thisbehavior is supposed to be caused due to thelowsolubilityofcarboninCu. Forthisreason Bae   and coworkers developed a roll-to-rollproduction of 30-inch graphene [1]. Using CVD, a 30-inch graphene layer was grown ona copper foil and then transfered onto a PETfilm by a roll-to-roll process. CVD also allowsa doping of the graphene, e.g. with HNO 3 , inordertodecreasetheresistance.  Bae   andcol-leaguesstackedfourdopedlayerofgrapheneonto a PET film and thus produced a fully functional touch-screen panel. It has about90% optical transmission and about 30 Ω persquare resistance, which is superior to ITO.4
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